System and method for generating random bit string in an integrated circuit

Present disclosure in general relates to Integrated Circuits (IC's), and more particularly, to generation of random bit strings in the IC's. The present application is based on, and claims priority from PCT application number PCT/IN2019/05022 filed on 20 Mar. 2019 an Indian Application No. 201821010427 filed on 21 Mar. 2018 the disclosure of which is hereby incorporated by reference herein.

Security and authentication has become an integral part in operation of today's smart devices. The smart devices are also implementing user friendly authentication while maintaining minimum financial burden on overall infrastructure. Each smart device is associated with a unique chip identification number for security. Uniqueness depends on randomness of the identification number which is of prime importance, so that security of the device should not be compromised.

In conventional methods, software-based algorithms are widely known for generating cryptographic keys. The cryptographic keys are stored in non-volatile memories (Read Only Memory (ROM), Flash, and alike). Generation of the keys through software-based algorithms may not be random in nature, as the keys are produced through codes or algorithms and may be cracked or compromised.

Furthermore, hardware-based security solutions rely on Physically Unclonable Functions (PUF) for generating keys through stochastic uncontrollable manufacturing variation. PUF based on frequency mismatch in ring oscillator, voltage mismatch between cross-coupled inverters in SRAM, stochastic nature of conducting filaments in RRAM, stochastic switching in MTJ has gained wide popularity, however, even for these listed PUF's, truly unbiased randomness remains a point of concern.

Bias in the PUF's may be corrected through one or more post-processing techniques. Examples of the post-processing may include NBTI aging for SRAM. However, the post-processing techniques are associated with complex algorithms and hence are not easy to implement, due to many hardware related constraints in the smart devices or Internet of Things (IoT) devices.

The principal object of the embodiments herein is to provide a system and method for generating a random bit string in an integrated circuit.

Another object of the embodiments herein is to provide a configuration of predefined number of One Time Programmable Memory (OTPM) devices connected in parallel for generating the random bit string.

Another object of the embodiments herein is to provide a breakdown of one of an OTPM device of the at least two OTPM device resulting in a broken OTPM device while leaving remaining OTPM devices of the at least two OTPM devices unbroken for generating the random bit string through at least one of the broken OTPM device and a remaining unbroken OTPM device of the at least two OTPM devices.

Accordingly, embodiments herein provide a system for generating a random bit string in an Integrated Circuit. The system comprises a predefined number of One-time Programmable Memory (OTPM) devices connected in parallel with each OTPM device of the predefined number of OTPM devices configured for producing a random bit-string. The system further comprises a current limiting circuit, connected in series with the at least two OTPM devices, and a voltage source, for supplying a predefined voltage to the at least two OTPM devices, such that the predefined voltage when supplied, produces a breakdown in one of an OTPM device of the at least two OTPM devices resulting in a broken OTPM device while leaving remaining OTPM devices of the at least two OTPM devices unbroken. The random bit string is generated through at least one of the broken OTPM device and a remaining unbroken OTPM device of the at least two OTPM devices.

Accordingly, another embodiment herein provides a method for generating a random bit string in an Integrated Circuit. The method comprises configuring, a predefined number of One-time Programmable Memory (OTPM) devices connected in parallel, with each OTPM device of the predefined number of OTPM devices configured for producing a random bit-string. The method further comprises connecting in series, a current limiting circuit with the at least two OTPM devices and supplying a predefined voltage, through a voltage source to the at least two OTPM devices, such that the predefined voltage when supplied, produces a breakdown in one of an OTPM device of the at least two OTPM devices, resulting in a broken OTPM device while leaving remaining OTPM devices of the at least two OTPM devices unbroken. The random bit string is generated through at least one of the broken OTPM device and a remaining unbroken OTPM device of the at least two OTPM devices.

These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. The term “or” as used herein, refers to a non-exclusive or, unless otherwise indicated. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein can be practiced and to further enable those skilled in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

Accordingly, embodiments herein provide a system and a method for generating a random number string in an Integrated Circuit (IC). The system comprises a plurality of One-time Programmable Memory (OTPM) devices connected in parallel. Random number may be generated by supplying a breakdown voltage such that any one of the OTPM device out of the plurality of OTPM devices will breakdown while leaving remaining OTPM devices unbroken. For generating the random number (random bit) with probability of ‘1’ i.e. p=1/n, or p=1−1/n (where n is a predefined number of the OTPM devices and n is greater than 1), n OTPM devices may be connected in parallel. After the breakdown, the broken OTPM device may be assigned a bit ‘1’ and the remaining unbroken OTPM devices may be assigned a bit ‘0’. The random bit string may be generated by selecting one of the broken OTPM device or the remaining unbroken OTPM devices.

Referring now to the drawings, and more particularly to, there are shown preferred embodiments.

According to an embodiment, referring to, a systemfor generating a random bit string in an Integrated Circuit (IC) is shown. The systemcomprises a predefined number (n) of One-time Programmable Memory (OTPM) devicesconnected in parallel. Each OTPM deviceof the predefined number of OTPM devicesis configured for producing a random bit-string (random number). The random bit string further produces a random code.

In, the predefined number of OTPM devicesare represented as R, R, R. . . R. The systemfurther comprises a current limiting circuitconnected in series with at least two OTPM devicesof the predefined number of OTPM devices. The current limiting circuitcomprises one of a poly resistor, a diffusor resistor, or a transistor switch.

A voltage sourceis configured for supplying a predefined voltage to the at least two OTPM devices. Switch Sto Sn comprises standard MOSFET switches. The switches Sto Sn are in ‘ON’ state when the systemis programmed, to allow breakdown of one of the OTPM device. After the breakdown one of the switch from the switches Sto Sn may be turned on for reading one of the OTPM device.

Referring to, detailed configuration of an OTPM deviceof the predefined number of OTPM devicesis shown. In an example, configuration of a single OTPM deviceis explained herein. The OTPM devicemay be implemented by using one of a MOSFET or a MOS capacitor in front end or stacked device enabled byD integration. The OTPM devicecomprises an anti-fuse device. The anti-fuse device comprises one of a gate oxide MOSFET or MIM device. The insulator material in MOSFETS may comprise one of a high-K material (potentially for advanced nodes) or SIO2/SiON based (potentially for lower nodes.

The MIM device may comprise one or more metals. The one or more metals comprise one of an Aluminium (Al), a copper (Cu) with thin insulator material like oxide like PECVD SiO2/HfO2/TiO2 sandwiched between two metals. The OTPM device(MIM OTPM device) may comprise first electrode placed on a substrate layer. An isolation layer is deposited on the first electrode. One or more Metal Insulator Metal (MIM) layer is deposited on the isolation layer and the first electrode. A second electrode is placed above the MIM layer.

In an exemplary embodiment, generation of the random number string through the systemis explained by considering a scenario of the at least two OTPM devicesconnected in parallel. It is to be understood that the random number may be generated by connecting the plurality of the OTPM devicesby selecting the predefined number ‘n’. The predefined number of OTPM devicesis selected to achieve a desired rate of probability of breakdown of the one of the OTPM devicesof the predefined number of OTPM devices. The OTPM deviceis generated based on a random breakdown event in oxides in one of the MOSFETs or MIM capacitor for generating the random binary code.

The voltage sourcesupplies a predefined voltage to the at least two OTPM devices. The predefined voltage is supplied according to a median breakdown voltage value of the system. Value of the predefined voltage is at least one of a larger than the median breakdown voltage value, smaller than the median breakdown voltage value or equal to the median breakdown voltage value. The median breakdown voltage refers to a voltage at which 50% of the OTPM devicesmay breakdown when a varying voltage stress is applied to the OTPM devices.

The predefined voltage is supplied to produce a breakdown in one of an OTPM deviceof the at least two OTPM devices. Initially the OTPM devicesare in non-conducting state (High Resistance State). When the predefined voltage (constant high voltage stress) is applied to the OTPM devices, the breakdown time for each of the OTPM devicewill be unique and random. The breakdown of the one of the OTPM deviceis associated with the breakdown time. Therefore, the breakdown of the one of the OTPM deviceis also random and may not be known a priori. The systemthus generates unbiased random binary bits and may withstand variation in voltage supply without affecting bias.

The breakdown results in a broken OTPM devicewhile leaving the remaining OTPM devicesof the at least two OTPM devicesunbroken. The broken OTPM devicewill be in conducting state (Low Resistance State) and may be classified as bit ‘1’. The unbroken OTPM device may be classified as bit ‘0’. The random bit string is generated through at least one of the broken OTPM deviceand a remaining unbroken OTPM deviceof the at least two OTPM devices. As shown in the, any one of the OTPM devicefrom the Rto Rmay become the broken device while the remaining OTPM deviceswill be unbroken OTPM devices.

Once the broken OTPM deviceis obtained after the breakdown, the current limiting circuitallows a predefined current to flow through the current limiting circuitafter the breakdown of the one of the OTPM device. The current limiting circuitfurther causes a potential drop across the current limiting circuitthat results in a negligible potential drop across the remaining unbroken OTPM device. The current limiting circuitprevents a breakdown of the remaining unbroken OTPM device.

The voltage sourcebrings the one of the OTPM deviceof the at least two OTM devicesin the conducting state after the breakdown. The systemfurther comprises a sense circuitry connected at a predefined point for sensing a state of one of the broken OTPM deviceand the remaining unbroken OTPM deviceof the at least two OTPM devices. In an embodiment, the sense circuitry is connected between the at least two OTPM devices. The sense circuitry comprises one of a comparator or a transistor

In an example embodiment, still referring to, for generating the random number with controllable bias the predefined voltage (high voltage stress) is applied to the “n” OTPM devicesconnected in parallel for a sufficient time. Since the breakdown time is random and unique for each of the OTPM deviceconnected in parallel, therefore, out of the “n” OTPM devices only one of the OTPM device will breakdown first. The breakdown will then reduce a voltage difference across the “n” OTPM devices-. Since the voltage is reduced across the n OTPM devices, there will not be any further breakdown due to the applied predefined voltage as voltage will get reduced across the parallel connection of the OTPM devices.

In an embodiment, the selection of the predefined number of the OTPM devicesis explained. The predefined number of OTPM devicesis selected to achieve a desired rate of probability of breakdown of the one of the OTPM devicesof the predefined number of OTPM devices.

In an example, referring to, the systemmay comprise of the two OTPM devices(Rand R) for generating the random number with percentage of ones=50%, R is the constant resistor connected in series (current limiting circuit) and Sand Sare nominal transistors. The predefined voltage may be applied across the two OTPM devices connected in parallel. Since, the systemcomprises only the two OTPM devices, a probability of breakdown of one of the OPTM device after the predefined voltage is applied will be 50% (1/n, here n=2). Out of the two OTPM devices, a leftmost OTPM device may be read by applying a read voltage (much lower than program voltage or the median breakdown voltage of the system) for generating the random number. For reading the leftmost device shown in, Switch S(nominal transistor) may be turned on and switch S(nominal transistor) may be turned off.illustrates a program current (median breakdown voltage of the system) verses time plot for the systemof.illustrates read current for the OTPM devices Rand R.

For generating the random number with percentage of ones=100/n, the n OTPM devicesmay be connected in parallel. For n=3, the probability of breakdown for the leftmost OTPM device is ⅓, for n=4 the probability of breakdown for the leftmost OTPM device is 25% and so on. Similarly, by increasing the predefined number ‘n’ by 1 from 2 to 10, the random number may be generated with percentage of the breakdown probability (probability of 1) varying from 50%, 33.33%, 25%, 20%, 16.66%, 14.28%, 12.5%, 11.11%, and 10%.

In yet another example, for generating the percentage of the breakdown probability to be greater than 50% probability, the complement of the number may be taken to achieve a probability of 1 to be 80%. The predefined number may be taken 5 and the 5 OTPM devices may be connected in parallel. Out of the 5 OTOM devices, the probability of breakdown for one OTPM device by applying the predefined voltage will be 20% and the probability for the OTPM devices of not being broken will be 80% i.e. percentage of zeros=80%. Therefore, the complement will be 80% for percentage of ones.

Referring to, generation of a single random bit number is shown. For generating “M” bits random number with probability of one, 1 (1 is the conducting state) of “p”=1/n or “p”=1-1/n, “M” (n>1) sets of “n” parallel OTP devicesand the current limiting circuitin series may be connected. A unit cell for generating the single random bit ofis further shown in

As shown in, “M” unit cell (shown as X. . . X) is connected for generating the “M” bit random number with probability of ones=1/n. Since there are P word lines, P sequence with ‘M’ bit strings may be generated, as shown in the. The number of the OTPM devicesconnected in parallel in the unit cell may be different for different plurality of word lines WL-to WL-p and plurality of bit lines (BL-to BL-m). During program, the voltage stress is applied through each of the word line of the plurality of Word Lines (WL-to WL-p) and the plurality of Bit lines (BL-to BL-m) are grounded. During read, a read voltage is applied through one of the Word line. Value of the read voltage is less than the median breakdown voltage value. The read voltage is applied for sensing the state of the one of OTPM devices. Through the read voltage, a conducting state or an insulating state of the one of the OTPM deviceis sensed and the ‘M’ bit random number is generated based on the sense of the state of the OTPM device.

In another embodiment, referring to, a methodfor generating a random bit string in an Integrated Circuit is shown. The methodmay be executed through the system.

At step, the predefined number of the One-time Programmable Memory (OTPM) devices are configured. The OTPM devicesare connected in parallel. Each of the OTPM deviceof the predefined number of the OTPM devicesare configured for producing the random bit-string.

At step, the current limiting circuitis connected in series with the at least two OTPM devices.

At step, a predefined voltage is supplied through the voltage source to the at least two OTPM devices. The predefined voltage when supplied, produces a breakdown in one of the OTPM deviceof the at least two OTPM devicesresulting in the broken OTPM devicewhile leaving remaining OTPM devices of the at least two OTPM devices unbroken. The random bit string is generated through the at least one of the broken OTPM deviceand the remaining unbroken OTPM deviceof the at least two OTPM devices.

It is to be understood that details of the methodare similar to the details of the systemare not repeated for the sake of brevity.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.